1. Field of the Invention
The present disclosure generally relates to telecommunication techniques and has been developed with particular but not exclusive attention paid to its possible application to telecommunication systems based upon the CDMA/3GPP TDD standard (where said acronyms stand for Code-Division Multiple Access/Third-Generation Partnership Project Time Division Duplex), in particular in its High-Chip-Rate option (operating at 3.84 Mcps).
Even though in what follows, for reasons of clarity and simplicity of exposition, practically exclusive reference will be made to the aforesaid application, it should, however, be borne in mind that the scope of the present disclosure is more general. The disclosure is, in fact, applicable to all telecommunication systems in which operating conditions of the type described in what follows occur: by way of non-exhaustive example, the satellite-telecommunication systems and the mobile cellular systems corresponding to the UMTS, CDMA2000, IS95 or WBCDMA standards may be cited.
2. Description of the Related Art
To enable acquisition of a base station by a mobile terminal included in a telecommunication system based upon the 3GPP standard TDD mode or the like, the corresponding receiver requires means capable of carrying out the function of frame synchronization and identification of the so-called codegroup. The possibility of performing the said functions is essential for execution of the subsequent steps in the context of the cell-search system.
In particular, when a mobile terminal is turned on, it does not have any knowledge of the timing of the transmitting cell to which it is to be assigned. The 3GPP standard, therefore, proposes an initial cell-search procedure for acquiring the cell signal and synchronizing therewith.
Said procedure basically comprises three steps, which in the TDD version are indicated as follows:                Primary Synchronization Code Acquisition (first step);        Code group (i.e., cell code group) Identification and Slot Synchronization (second step); and        Downlink Scrambling Code, Basic Midamble Code Identification and Frame Synchronization (third step).        
In the implementation of the second step described above, it is assumed that the primary synchronization code, which indicates the position of the generic frame of the synchronization slot has previously been acquired during the first step.
To obtain at this point the slot synchronization and identify the codegroup, to which the offset of the cell is associated, in the second step the Secondary Synchronization CHannel (SSCH) is used, on which there are transmitted, in each synchronization slot, three secondary synchronization codes or words of 256 chips (i.e., letters), where the generic code is designated by Cm, with m ranging from 0 to 15.
In the TDD version, as mentioned previously, in each slot containing the secondary channel SCH there are three secondary synchronization codes SSC. The standard defines four possible sets of three codes; the combination of the set of three codes SSC and their phases defines the codegroup. Each codegroup identifies: one slot offset between the start of the slot and the start of the code on the secondary channel SCH; four possible basic midambles; and one scrambling code, associated to each of said midambles. In the third step of the cell search, there is defined which midamble is used in the primary channel P-CCPCH (Primary Common Control Physical CHannel) and hence the corresponding scrambling code. In the third step, in the received signal the portion of slot of the primary channel P-CCPCH where the midamble falls is considered, and on said portion there is performed a correlation with each of the four possible midambles known to the receiver and there is defined which midamble has been transmitted according to the correlation results. Normally, the midamble which has the highest correlation energy is chosen.
The detection of the midamble is moreover necessary for executing a channel-estimation procedure.
To carry out channel estimation in the TDD 3.84 mode, the sequence present in the midamble can be used (i.e., a numeric sequence of values ±1, ±j), which is known to the receiver and is sent in the generic data slot DPCH (Dedicated Physical CHannel) between two data blocks referred to as Data1 and Data2.
The rules of construction of the generic midamble are, however, described in the 3GPP standard. From a basic midamble sequence of length Lbasic=KW, where K is the number of users and W a known quantity, each user midamble of length Lmidamble can be constructed by carrying out an operation of shift of the basic midamble sequence itself by the known amount W.
The midamble sequence received at the user terminal, designated in what follows as emidamble, is the convolution between the transmitted midamble sequence, mid, which is also the midamble sequence known to the receiver, and the impulse response of the transmission channel h(t):emidamble=mid*h(t)  (1)
Since the transmitted midamble sequence mid is known to the receiver, the receiver can reconstruct, from the received signal, the channel impulse response h(t), i.e., its coefficients.
In the case of down-link transmission, i.e., of transmission from the base station to the mobile user terminal, the signals of the generic K users are transmitted simultaneously by the base station. In this case, the channel estimation to be performed in the user terminal can be made using just one midamble sequence, i.e., the j-th one. In this case, the vector of the channel estimates hw becomes:hw=[h1(j), h2(j), . . . , hw(j)]  (2)
Furthermore, it is important to consider the fact that the channel introduces a delay, or, rather, it introduces signal “tails”, which are estimated as corresponding to an impulse response of W chips. It is clear that the first (W−1) chips of the received midamble emidamble are affected by the intersymbol interference of the last (W−1) chips of the preceding data block Data1. Only the sequence of subsequent chips of length. L=KW can in actual fact be used for performing the estimation of the impulse response of the channel.
Assuming that the characteristics of the radio channel are invariant, or at least vary little, during a time interval of one slot, a simple method for obtaining the vector of channel estimates hw, for the j-th user, is the cross-correlation between the last chip sequence of length L=KW of the received midamble and a set of delayed replicas of the expected or known midamble, which is generated locally by the user terminal and which is designated, as a whole, by mid like the transmitted midamble. The results of these cross-correlations are the elements or channel coefficients sought and make up the so-called channel profile (which can be reduced to just W values).
A possible improvement in the channel estimation can be obtained by averaging on a number of windows having W elements, each obtained via cross-correlation between the last L=KW chips of the received midamble emidamble and the delayed replicas of the transmitted or known midamble mid of each user that is active in that precise slot within the service cell.
In the techniques known to the state of the art, the steps corresponding to the cell-search procedure and to the channel-estimation procedure are implemented using different subsystems based upon separate sections of correlation, which obtain, in a cascaded way, the parameters of interest.
The first subsystem performs the second step of the cell search, extracting all the parameters required by the synchronization procedure of the standard along with the information useful for execution of the subsequent third step of the cell-search procedure, in particular, the codegroup. Said third step, according to the information already obtained, carries out appropriate processing on the received signal. Said processing normally consists of a correlation between the received signal of an appropriate section of the synchronization slot representing the midamble of the primary channel P-CCPCH and the four possible midamble codes, according to the parameters estimated in the second step.
The channel-estimation procedure, or, more precisely, the procedure of estimation channel of the coefficients to be used then in the procedure of demodulation of the data portion, is carried out normally on the midamble of the generic received slot. Said procedure has many points in common with the third step of the cell-search procedure. The differences lie in the fact that the generic data channels can make use of two types of midambles of different lengths, a first, long, midamble MPL of 512 chips, which is equal to the one used in the primary channel P-CCPCH, and a second, short, midamble MPS of 256 chips. It is to be noted that the type of midamble used in the generic data burst is determined during the cell-search procedure in the second and third steps via appropriate tables defined by the 3GPP standard TDD mode. The channel P-CCPCH always uses a long midamble (512 chips).
Furthermore, in the case of the channel-estimation procedure, it is necessary to correlate the received midamble emidamble, or part thereof, with a certain number of delayed replicas of the expected midamble mid. Said operation of correlation with delayed replicas enables definition of the complex weights or coefficients characterizing the possible paths present on the channel.
FIG. 1 shows a parallel-architecture device for implementing the second step of the cell-search procedure in the TDD mode, known from the Italian patent application TO2002A001082, filed in the name of the present applicant.
The received signal r goes at input to a block designated by 110, which executes a first operation of correlation on a first sequence of 16 chips.
The received signal r at output from block 110 is sent to a bank of correlators 111, which constitutes the correlation section. The samples of the received signal r are moreover stored in a storage unit 112.
The bank of correlators 111 comprises just four correlator circuits, one for each code set. The bank 111 receives, in fact, from a code-generating system designated by 113, four “first” codes SSC, each belonging to and identifying one of the four possible code sets in the context of the set of codes SSC.
Such codes SSC are in number of twelve, and each code set comprises a “first” code, which identifies the set, and a subset of remaining codes, which comprises the other two codes of the set.
The operation of correlation carried out in block 111 is hence able to supply at output an estimate of the received code set.
In this regard, a unit for search of the maximum value, designated by 114, receives from the bank of correlators 111 the energies corresponding to the four first codes SSC supplied by the system and supplies at output a first code SSC provided with the best correlation energy, along with the corresponding phase offset. In this way, there is hence identified a code set CS, to which said first code SSC belongs.
The first code SSC and its phase offset are designed to be sent on to a comparison block 115, whilst the information on the code set is sent to a controller designated by 116.
The controller 116 supervises operation of the circuit and, in particular, is designed to supply, to the code-generating system 113, the information on the four “first” codes SSC to be generated for identifying the four code sets.
On the basis of the first code and of the corresponding code set CS identified via the search for the maximum carried out in unit 114, the controller 116 sends, to the code-generating system 113, the information on which other codes SSC are to be generated for the operation of correlation with the received signal r, stored in the storage unit 112.
Said other codes are simply the two remaining codes SSC comprised in the subset that completes the code set corresponding to the first code selected via the search performed in the unit 114.
Provided upstream of the bank 111 is a multiplexer 120, which, driven by the controller 116, selects the output of the block 110 or else the output of the storage unit 112, in order to send it to the bank 111.
In this way, initially, in addition to being stored in the unit 112, the received signal r is sent directly to the block 111, where it is correlated with the four “first” codes that identify the four code sets coming from block 113.
Subsequently, once the reference code set has been identified—as a result of the search performed in the unit 114—, the samples of the received signal r stored in the unit 112 can be sent to the block 111 for being correlated with the two remaining codes of the aforesaid code set.
The bank of correlators 111 is provided with a correlator memory 121, in which the first code SSC of the detected code set is stored.
According to the information corresponding to the selected code set CS, the controller 116 commands the code-generating system 113 to generate the two codes corresponding to the two codes that are missing to compose the set of three codes of the code set CS, in order to perform a correlation with the samples of the received signal r stored in the storage unit 112.
The result of this correlation operation (carried out, so to speak, “recycling” two of the correlators contained in the bank 111) is also supplied to block 115, where there is recomposed the set of three codes of the code set CS, which can be used—along with the corresponding phases—for the comparison with the standard tables in order to extract the corresponding parameters from the table contained in the comparison block 115.
FIG. 2 shows a parallel-architecture device of a known type for implementing the third step of the cell-search procedure in TDD mode, which envisages sending the received signal r to the input of a block for extraction of the midamble 150, which extracts the part of slot in the received signal r that contains the received midamble emidamble. Placed downstream of said block for extraction of the midamble 150 is a block of correlators 151, comprising four correlators. For the third step of the cell-search procedure, it is assumed that all the necessary information, among which the codegroup-CD, has been received from the previous step. According to said information, the part of received signal r is correlated by the block of correlators 151, within the appropriate time window, which is selected via the block for extraction of the midamble 150, with the local replicas of the four possible expected midamble codes MPL, which belong to the codegroup identified previously. Said four possible midamble codes MPL are supplied in parallel by an appropriate midamble-code generator 152. The resulting correlation energies are supplied to a unit for detection of the maximum value 153, which identifies the code provided with the highest correlation energy and sends it to a comparison block 155, in which, through a search in an appropriate look-up table, scrambling codes SCR corresponding to the cell are obtained. The reference number 156 designates a controller for handling the operations of the midamble-extraction block 150, of the midamble-code generator 152, and of the comparison block 155.
The sequence or burst associated to the primary channel P-CCPCH on which the third step of the cell-search procedure is to be performed is transmitted in concomitance with the first secondary channel SSCH of each frame.
Hence, the solutions according to the known art require allocating a certain amount of memory and, consequently, area of chip, with a consequent power consumption for implementing the sections of correlation necessary for the cell-search procedure and for the channel-estimation procedure.